Summary Currently we are faced with a significant conundrum with regard to the opioid crises. On the one hand, we have no viable alternative for treating severe post-surgical and chronic pain. On the other hand, people taking these drugs for legitimate purposes are transitioning at an alarming rate to opioid overuse and abuse. While the dramatic increase in opioid abuse has coincided with the much-reported increase in opioid prescriptions, it is still true that only a fraction of patients who are prescribed opioids for pain transition to abuse. If we could identify those pain patients most at risk prior to opioid treatment, we could potentially stem the tide of opioid abuse. A number of compelling studies have recently shown that alterations in the gut microbiome can influence a plethora of central nervous systems disorders, leading to widespread acceptance of the concept of a gut-microbiome- brain axis. We find it compelling that, coincident with the increase in opioid prescriptions, there has also been a large increase in the routine and often unnecessary use of oral antibiotics?a trend that that has altered the gut microbiome of an entire generation. Not surprisingly, given the widespread expression of opioid receptors in the gut, opioids have been shown to alter the gut microbiome. Furthermore, alterations in the microbiome have been shown to alter opioid analgesia and reward, suggesting that opioids influence the gut-brain axis. However, to date, no studies have examined whether the gut microbiome influences abuse liability. Here we will capitalize on both behavioral variability in a mouse model of compulsive drug seeking and relapse, and mouse microbiome variability, both innate and in response to morphine, to determine whether abuse liability and the microbiome co- vary (Aim 1). We will also examine whether the microbiome from an ?addict? mouse or ?non-addict? mouse can predispose to or protect subjects from compulsive drug seeking or relapse (Aim 2). This project is a dual Principal Investigator proposal that capitalizes on the complementary expertise of two experts in their respective fields, one a molecular microbiologist the other an opioid pharmacologist.